This invention relates to near field optical microscopy, and more particularly to a method and apparatus which eliminates the requirement for and hence the optical limitations of an aperture in near field optical microscopy. As a result, resolution of 1 nm or smaller using visible light wavelengths is obtainable.
Efforts to date in the art of near field optical microscopy have centered upon the generation of ultrasmall apertures on transparent tips or flat surfaces. Such work is described in the article entitled "Optical Stethoscopy: Image Recording With Resolution g/20" by D. W. Pohl et al, Appl. Phys. Lett. Vol. 44, No. 7, Apr. 1984, pp. 651 to 653 and in "Scanning Optical Spectral Microscopy with 500.ANG. Spatial Resolution" by A. Lewis et al, Biophys. Journal, Vol. 41, 1983, p. 405a.
The ultrasmall apertures generate a sub wavelength source of optical power which can be used to image surfaces with sub wavelength resolution when the aperture is scanned in close proximity to the aligned surface. There are two inherent limitations to the use of such apertures. First, the structures must be physically built onto a small tip (sub micrometer) when non-flat surfaces are to be imaged. Constructing such structures have proven to be technologically challenging. Second, there is a theoretical limit. In order to achieve the highest resolution, the aperture size should be reduced toward atomic (on the order of 1 nm) dimensions. However, the improvement in resolution does not steadily improve as the aperture size decreases. This effect is due to the aperture material having a finite absorption length, typically greater than 100 angstroms. The ultrasmall apertures, therefore, are not useful when the dimension sought to be resolved approaches the absorption length.
The present invention obviates the requirement of an aperture in near field optical microscopy by using a metallic or dielectric tip having very small dimensions, on the order of atomic dimension. Such a tip is found, for instance, in a scanning thermal profiler, scanning tunneling microscope, atomic force microscope or the like. A light beam illuminates the tip and a portion of the light striking the tip scatters and forms local evanescent fields from the very end region of the tip to the sample surface which is in proximity to the tip. The evanescent fields very close to the tip will interact with the surface atoms of the sample. By applying a first dither motion at a first frequency to the tip in a direction normal to the plane of the sample surface and applying a second dither motion at a second frequency to the sample in a direction parallel to plane of the sample surface, a detector is able to receive surface image signals at a difference frequency of the two dither frequencies having a high resolution without the background signal overwhelming the desired image signal. In an alternative embodiment, the tip is held stationary and the sample is made to undergo motion in both of the above mentioned directions each at a different frequency. The essence of the present invention is the obviating of the requirement of an aperture in a near field optical microscope and processing of the surface image signals to remove the adverse affects of the background signal.